ABSTRACT Akt1, Akt2 and Akt3 are members of a family of serine/threonine kinases that are key effectors of PI3K. The Akt family has been well characterized to promote cell survival, glucose metabolism and proliferation in many cell types including developing T cells. Thymocytes express all Akt isoforms to varying degrees. Ablation of single or multiple Akt genes results in differential blocks in T cell development. Loss of all three Akt isoforms leads to an early developmental defect due to decreased survival of CD4?CD8? double negative (DN) thymocytes. Expression of Akt2 or Akt3 alone is sufficient to preserve DN survival; however, DN3 differentiation and DN4 thymocyte proliferation remain defective along with survival of cells at later developmental stages. These distinct phenotypes suggest either that different Akt isoforms regulate distinct functions or that different doses of Akt activity are required to promote stage-specific cell survival, proliferation and differentiation. With no current evidence for isoform-specific functions, we focused our attention on characterizing mechanisms for regulating the dose of Akt activity. Surface receptors, including (pre-)T cell receptor and cytokine receptors, activate PI3K to generate PIP3. Interactions between PIP3 and the Pleckstrin homology (PH) domain of Akt are required to recruit Akt to the plasma membrane for activation. Thus, fine control of PI3K activity and PIP3 availability can directly regulate Akt activation levels. This application addresses an increasingly important alternative mechanism for indirectly controlling Akt activity: via generation of IP4, a soluble inositol polyphosphate that is structurally similar to PIP3. IP4 is generated following (pre)TCR stimulation and competes with PIP3 for binding to the Akt PH domain. Thymocytes deficient in IP4 activate Akt excessively following expression of preTCR at the DN3 and DN4 stages. Strikingly, Akt hyperactivity leads to accelerated ? selection, premature reprogramming to glycolytic metabolism and loss of Notch dependency. This leads us to propose the global hypotheses that 1) preTCR- induced IP4 generation restricts Akt-dependent metabolic reprogramming and proliferation to ensure adequate Notch signaling and 2) preTCR and Notch cooperative signaling is required for T cell lineage specification. Successful completion of this study will shift our current understanding of T lineage specification by revealing 1) an unexpected requirement for preTCR in imposing a Notch checkpoint and in co-stimulating Notch function, 2) a novel preTCR feedback mechanism that controls DN thymocyte proliferation/maturation via tuning of Akt activity and metabolic reprogramming; and 3) a non-canonical pathway for activating TCF1-dependent transcription to specify T cell commitment.